Light-driven integration of diazotroph-derived nitrogen in euphotic nitrogen cycle

Algae biofilm produces less microbe-derived dissolved organic nitrogen under higher C/N ratio conditions

The increased release of microbe-derived dissolved organic nitrogen (mDON) during biological nutrient removal (BNR) processes, particularly under carbon dosing conditions, has emerged as a primary cause to eutrophication. Although algae biofilm (AB) has potential in mitigating mDON discharge, the influence of wastewater carbon-to-nitrogen (C/N) ratios on mDON formation remains poorly understood. Here, we investigated AB's mDON formation and utilization performance, molecular characteristics, and metabolic traits under C/N ratios ranging from 1 to 8. All AB reactors reached mDON concentrations <1.3 mg/L, presenting a trend of first rising and then falling as C/N ratios rose. At the highest C/N ratio, AB effectively reduced mDON concentrations to 0.88 ± 0.08 mg/L, representing a reduction greater than 50 % compared to conventional BNR processes, and achieved a total nitrogen removal efficiency of 97.19 %. Redundancy and network analysis revealed that dominant algae (Chlorophyta and Cyanobacteria) and bacteria (Bacteroidota and Proteobacteria) exhibited distinct mDON production and utilization patterns across different C/N ratios. Algae proliferated under higher C/N ratios promoted the synergistic algal-bacteria interactions, enabling labile DON recycling and reducing its chemodiversity. This was also supported by the increased genetic investments in DON metabolism under higher C/N ratios. Conversely, bacterial activity, responsible for diversifying mDON pools via cross-module transformation reactions, was inhibited under elevated C/N ratios. Overall, AB is demonstrated robust for DON-related eutrophication control, even under high C/N ratios. This study first investigates the effects of C/N ratios on the mDON fates within algae biofilm systems and reveals the taxon-specific formation and utilization patterns.